A voltage-source converter (VSC) of high enough power rating may cause destabilization of grid resonances in the vicinity of the synchronous (fundamental) frequency f1. Of particular concern are subsynchronous torsional resonances. A torsional resonant frequency fr in the range of tens of hertz (Hz) is mirrored to the frequency f1-fr in the three-phase domain (as well as in the equivalent two-phase domain domain). It thus remains subsynchronous, i.e., it appears below the synchronous frequency. Often, the inherent damping of the system is sufficient to avoid resonance destabilization, but in some cases subsynchronous damping controllers are needed. In weak grids, the lowest electrical grid resonance may be close to, but above, the synchronous frequency, i.e., it is supersynchronous. Also in this case, destabilization may occur.
By “near-synchronous” we here imply a resonant frequency that may be subsynchronous or supersynchronous, but in the latter case typically not larger than 2f1.
A subsynchronous damping scheme, shown in FIG. 3, has been proposed by K. M. Alawasa, Y. A.-R. I. Mohamed, and W. Xu, in “Active mitigation of subsynchronous interactions between PWM voltage-source converters and power networks”, IEEE Trans. Power Electron., vol. 29, no. 1, pp. 121-134, January 2014. The damping scheme comprises a phase-locked loop (PLL) and is effective for damping of torsional resonances.